Lithium secondary battery is an energy storage device storing therein energy produced while lithium ions move from an anode to a cathode in a discharge process and from a cathode to an anode in a charge process. Compared with other batteries, lithium secondary batteries have been used in various industries since they have a high energy density and a low self-discharge rate.
A lithium secondary battery may include a cathode, an anode, an electrolyte, and a separator, and the like. While in an initial lithium secondary battery, a lithium metal is used as an anode active material, as safety problem due to repetition of charge and discharge appears, the lithium metal is replaced with a carbon-based material such as graphite. Since the carbon-based anode active material has an electrochemical reaction potential with lithium ions that is similar to lithium metal, and has a crystal structure that is less changed in the course of continuous intercalation and deintercalation of lithium ions, it can continuously charge and discharge the battery to improve charge and discharge life cycle.
However, as market recently expands from a small-sized lithium secondary battery used in portable devices to a large-sized secondary battery used in automobiles, high capacity and high power techniques of an anode active material are required, and thus development for non-carbon-based anode active materials such as silicon, tin, germanium, zinc, and lead having a higher theoretical capacity than carbon-based anode materials is in progress.
Among such non-carbon-based anode active materials, since a silicon-based material has 11 times greater theoretical capacity (4190 mAh/g) than the theoretical capacity (372 mAh/g) of the carbon-based anode active material, it is in the limelight as a material for replacing the carbon-based anode active material. However, in the case where silicon is used alone, when lithium ions are intercalated, since the silicon expands three times or more in volume, there appears a tendency that the battery capacity decreases as charge and discharge are repeated, and safety decrease too.
In recent years, studies on a silicon-based composite in which the silicon-based material and a carbon-based material are used together are actively in progress so as to minimize the volume expansion of the silicon-based materials and thus obtain a high capacity and increase charge and discharge cycle.
The most basic method for synthesis of a composite is a method in which carbon is coated on a silicon-based material. It is known that the silicon-based composite obtained by such a method enhances electrical conductivity between active material particles, and electrochemical characteristics with respect to electrolyte, and decreases volume expansion of silicon-based particles to increase the battery life.
However, when the silicon-based composite is used as an anode active material, the initial efficiency of the secondary battery may be reduced due to the formation of non-reversible phase by the silicon-based material in the initial charge and discharge.
Therefore, it is necessary to develop a method of producing a novel silicon-based composite that can overcome all the above-described limitations.